There appear to be two different schools of thought as to what drove the transformation we viewers have witnessed in the character of Shane Walsh between seasons 1 and 2. While I don't think anybody can debate his status as a certified badass, there is disagreement on whether Shane was adapting to his new post-apocalyptic environment in an effort to survive, or was simply losing his sanity.
SPOILER ALERT - The following contains references to episodes including and after "Pretty Much Dead Already".
Personally, I was a subscriber to the first theory right up until the scene in "Pretty Much Dead Already" when Shane broke open the barn full of Walkers.
I don't think I've ever done a 180 on my opinion of a character quite so fast. Right up until then, Shane appeared to be acting like a rational human being who had been knocked down several levels on Maslow's hierarchy of needs due to the collapse of civilization and the rise of the walking dead. He appeared to be willing to do whatever it took to survive and to keep Lori and Carl (to whom he felt some level of connection) alive. (Even kill his best friend, Rick.)
Then, he did something that made absolutely no rational sense to a survivalist; he deliberately unsecured a barn containing an undisclosed number of walkers and unleashed them upon himself and his companions, thus deliberately placing himself (and Lori and Carl, whom he claimed to be determined to protect) in clear and present danger for no logical reason. I cannot accept such a suicidally stupid action as that of a sane man.
That being said, I am of the opinion that Shane wasn't toughening up, he was gradually losing his mind. He wasn't becoming stronger, he was breaking down under the pressure. When he pried open that barn, he proved to the entire group that he was mentally unstable and a constant danger to all those around him.
That's my two cents, I want to get a debate going on this. If you have a theory, please weigh in.